1. Field of the Invention
The present invention relates to electronics and, more specifically, to circuits for implementing clock-frequency spreading.
2. Description of the Related Art
Spread-spectrum techniques in general are methods, in which energy generated in a narrow frequency band is deliberately spread over a wide frequency band. Frequency spreading can be done for a variety of reasons, e.g., to improve circuit robustness to electromagnetic interference (EMI) or jamming and to prevent hostile detection. In addition, as the speed of operation and degree of integration of electronic devices increase, the wavelengths of electromagnetic radiation generated within the electronic device decrease accordingly and become comparable with the size of wiring structures. As a result, the wiring structures can act as antennas capable of efficiently radiating and/or receiving unwanted electromagnetic radiation, which can cause high levels of EMI, severe deterioration of signal-to-noise ratios, and/or device malfunctions.
One spread-spectrum technique, called spread-spectrum clock generation (SSCG), is used in the design of synchronous digital circuits. A synchronous digital circuit is driven by a clock signal that provides for timing and synchronization. Typically, clock signals cause electromagnetic radiation on a plurality of relatively narrow bands located at the fundamental clock frequency and its harmonics. If left unabated, radiation levels might exceed the regulatory limits for EMI specified, e.g., by the Federal Communications Commission (FCC) in the United States, Japan Electronics and Information Technology Industries Association (JEITA) in Japan, and the International Electrotechnical Commission (IEC) in Europe.
With SSCG, the operating clock frequency is slightly varied to spread the radiated energy over one or more relatively wide frequency bands. The frequency variation is usually achieved through frequency or phase modulation, with the amount of EMI depending on the amplitude, frequency, and/or shape of the modulation signal. With the use of an appropriate modulation signal, the EMI level at any particular frequency can be reduced to an acceptable level. Representative prior-art circuits for implementing SSCG are disclosed in U.S. Pat. Nos. 7,098,709, 7,095,260, 7,015,733, 6,798,303, 6,687,319, and 5,226,058, all of which are incorporated herein by reference in their entirety. However, one problem with prior-art SSCG circuits is that they often draw a relatively large current, which might be disadvantageous, e.g., for portable cellular devices.